Composition containing fine silver particles, production method thereof, method for producing fine silver particles, and paste having fine silver particles

ABSTRACT

A composition containing fine silver particles which have a uniform particle size, can form a fine drawing pattern, and have a small environmental impact, a method for producing that composition, a method for producing fine silver particles, and a paste having fine silver particles are provided. The fine silver particles are produced by carrying out a fluid preparation step of preparing a reduction fluid, a silver reaction step, and a filtration/washing step. The reaction step is carried out by adding an aqueous silver nitrate fluid to a reduction fluid whose temperature has been increased to a range between 40 and 800° C. The aqueous silver nitrate fluid is added at a stretch. The composition containing fine silver particles is produced by dispersing the composition containing the fine silver particles in a polar fluid.

TECHNICAL FIELD

The present invention relates to a method for producing fine silverparticles which have diameters a less than 100 nm, a compositioncontaining of those fine silver particles, a method for producing suchcomposition, and a paste having fine silver particles.

BACKGROUND ART

Materials which diameters are less than 100 nm (it paraphrases it inthis description like this from now on; “nano-sized substance”) areknown to have a large specific surface area and to exhibit differentcharacteristics from those of substances which are generally known.

In particular, metals are found to exhibit phenomena, because ofreactivity increases at the surface of the fine particle, melting pointis decreased.

Because of their unique characteristics, nano-sized substance ispromising at the field of electronic devices; the production ofequipments to small size, and the provision at low cost.

Downsizing of electronic devices are mainly blessed with improvements inthe degree of integration of semiconductors.

Furthermore, higher integration of the pattern connecting between thesemiconductors on a printed substrate is also necessary for thedownsizing of electronic devices.

It is desirable for such a highly integrated pattern to have a narrowline width and a high electrical conductivity.

It was able to overcome such demands; they were able to use somemethods, for example, plating, vapor deposition and sputtering.

However, we need to prepare big apparatus when we'll use those methods.And we have to renew when we need to more fine patterns, so they aren'tsuitable for drawing of finer lines.

Thus, if we can get apparatus which has compact and high efficiency,it's very comfortable to draw of fine metallic lines.

Examples of methods for forming a drawing pattern which satisfy theserequirements include methods employing a printing technique, such asinkjet printing and screen printing.

For example, to form a drawing pattern on the substrate by a printingtechnique, a conductive paste in which a fine metal powder is dispersedin a binder is used.

However, problems with this method are that the precision of the patternformation is low, and that the electrical conductivity of the producedpattern is lower than that of the original metal.

Conventional conductive pastes, which dispersed in an organic resinbinder, contains of metal particles which average diameter is fromseveral tens to hundreds microns. Thus, it is difficult to form fineline more than a diameter of average particle size.

Furthermore, the printed pattern has a structure in which the metalpowder contacts each other at points. Consequently, the electricalconductivity is substantially worse than that of a sheet shape of thepure metal.

One way to resolve such problems of a conductive film formed by printingis to use the above-described nano-sized metal particles.

If such fine metal particles are used, a high-precision drawing patterncan be formed at a higher density than conventional paste. Consequently,we can get increasing the number of particles per unit volume; soelectrical conductivity can also be expected to improve to closer tothat of a sheet shape of the pure metal.

In addition, because of increases of the fine particle reactivity,melting point is decreased. Because of those phenomena, we can get asheet of metallic at lower sintering temperature than conventional.

From this perspective, various methods have been investigated which wasusing nano-sized metal particles, and several proposals have alreadybeen made.

As methods for producing nano-sized metal particles, gas-phase methodsand fluid-phase methods are mainly known.

For example, Patent Document 1 describes that a dispersion ofindependent ultra fine silver particles can be obtained by producingultra fine particles of silver by a gas-phase method in vacuo and thenmixing these particles with an organic solvent. In this dispersion, thesurface of the ultra fine particles is covered with the organic solvent,so that each of the particles is independently dispersed.

Patent Document 2 describes ultra fine particles obtained by afluid-phase method.

In this technique, metal fine particles formed by reduction of metalions in an aqueous phase undergo a phase transfer from the aqueous phaseto a more stable organic solvent phase.

More specifically, a small amount of a protective colloid is made to bepresent in the organic solvent in advance so that the metal fineparticles undergo a phase transfer from the aqueous phase to the stableorganic solvent layer, and are formed as stable colloid particles. Thisallows the metal particles to be obtained in a high concentration in theorganic solvent phase.

Patent Document 3 also describes a production method in the fluid phase.

Patent Document 3 describes that during the production of silvernano-particles by reducing a silver salt in a solvent, rather than thetypically used silver nitrate, a silver halide (especially silverchloride or silver bromide), which is insoluble as a silver salt, isused. Furthermore, in the method described in Patent Document 3, thereduction is carried out in the presence of a protection agent formedfrom a compound which is dissolved in the solvent and which can becoordinated with silver.

In this method, it is described that a mono-dispersion can be obtainedin which silver nano-particles are coated/protected by the protectionagent and dispersed in the solvent.

It is also described that a polar solvent is used as the solvent, and athiol, such as thiocholine bromide, is preferred as the protectionagent.

Patent Document 4 also describes a method for obtaining nano-order finesilver particles which are mono-dispersed in a polar solvent using afluid-phase method.

This document describes that silver nitrate is used as the startingmaterial for obtaining the fine particles of silver, and that heptanoicacid is used as the protection agent.

-   [Patent Document 1] Japanese Patent Application Laid-Open No.    2001-35255-   [Patent Document 2] Japanese Patent Application Laid-Open No. Hei.    11-319538-   [Patent Document 3] Japanese Patent Application Laid-Open No.    2003-253311-   [Patent Document 4] US 2007/0144305 A1

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

A conventional conductive paste contains a resin as a binder forfastening metal fine particles after formation of a pattern by printing.

To include the resin, an organic solvent is used as a solvent.

However, organic solvents have some problems, such as the ventilation isrequired during operation, and the combustion treatment of solvent isnecessary during sintering, and the waste fluids producing by washingcannot be disposed of into the environment. Because of these problems,the organic solvent has drawbacks in terms of working efficiency,safety, and impact on the environment.

As a method for producing metal fine particles, from the perspective ofindustrial mass-productivity, a fluid-phase method is appropriate.

The method of these fluid-phase, it is preferred that the metal fineparticles can be synthesized in a polar solvent, particularly water. Andit is more preferred that the synthesized metal fine particles can beeasily collection and are stably present even in a dry state. Andmoreover it is preferred that the obtained dry state particles can beeasily redispersed by the polar solvent.

Although Patent Document 3 or Patent Document 4 can produce such metalfine particles, the obtained fine particles often have an unstableparticle size depending on the production conditions.

Furthermore, the dry state metal fine particles obtained by thesynthesis described in Patent Document 4 require further improvement interms of their redispersibility in the polar solvent.

The present invention was devised in view of such problems in theconventional art. It is necessary of solving by the present invention toprovide are these, a first purpose is the present invention is a methodof producing fine silver particles which have a uniform particle size,and second purpose of the invention is a method of producing compositionof fine silver particles and the composition thereof, and third purposeof invention is getting of a paste of fine silver particles. And thesesolving methods need with small environmental impact.

Means for Solving the Problems

For a result of extensive research to achieve the above-describedobjects, inventors discovered that the objects could be achieved bysynthesizing silver particles in a fluid phase, and then dispersing theobtained particles by a polar solvent, thereby arriving at the presentinvention.

More specifically, a composition containing fine silver particlesaccording to the present invention is characterized by including silverparticles which was attached to a linear fatty acid having 6 or lesscarbon atoms.

Furthermore, a method for producing fine silver particles by presentinvention is including these characteristic steps; a step of preparing areduction fluid containing water, ammonia water, hexanoic acid, and anaqueous fluid of hydrazine hydrate; a step of reduction, an aqueoussilver nitrate fluid adding into the reduction fluid; and a step offiltering a product of the reaction and then washing the product withwater. And a method for producing composition of fine silver particlesaccording to the present invention further includes this step,dispersing the fine silver particles, which produced by theabove-described method, into a polar solvent.

In addition, a paste according to the present invention is characterizedby having the fine silver particles included in the compositioncontaining fine silver particles.

Effects of the Invention

By using a linear fatty acid having six or less carbon atoms as aprotection agent, the present invention can provide a method forproducing fine silver particles which can exist even into a polarsolvent including water, and a composition containing fine silverparticles, and a producing method for such composition, and a pastehaving fine silver particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows SEM photographs of an example according to the presentinvention.

FIG. 2 shows SEM photographs of Comparative Example 2.

FIG. 3 shows SEM photographs of a comparative example in which aprotection agent is not used.

FIG. 4 shows TEM photographs of an example according to the presentinvention.

FIG. 5 shows TEM photographs of a comparative example in which heptanoicacid was used as a protection agent.

FIG. 6 shows TEM photographs of Comparative Example 2.

FIG. 7 shows TEM photographs of a comparative example in which aprotection agent is not used.

FIG. 8 is a graph showing the relationship between the solvent anddispersibility.

FIG. 9 is a graph showing the relationship between pot life andviscosity.

BEST MODE FOR CARRYING OUT THE INVENTION

In the description of the present invention, the term “agglomerated”means to a state in which a plurality of particles are agglomerated witheach of the particles present as individual particles, and each of theparticles can again be dissociated by a suitable dispersion treatment.Furthermore, the term “aggregated” means to a state in which a pluralityof particles are fused together to form a single rough particle whichcannot again be dissociated even if subjected to a dispersion treatment.

The composition of present invention is fine silver particle covered ina fatty acid having six or less carbon atoms. Due to this configuration,the fine silver particles in agglomerate state exist stably underaridity. Furthermore, this agglomeration dissociates into individualparticles, they can exist stably in the polar solvent.

The linear fatty acid having six or less carbon atoms functions as aprotection agent.

This protection agent has a function which exists with stability,because of covering the silver particles, or in other words, by allowingto adhere to the surface of the silver particles, to avoid fusing witheach other.

In the present invention, a comparatively linear fatty acid with alittle numbers of carbons is preferred.

Specifically, it is preferred to use hexanoic acid.

In addition, the composition containing fine silver particles of thisinvention may be having polar solvent. In this case, the fine silverparticles can be dispersed into this polar solvent.

Water or an organic solvent having a polar group can be used as thispolar solvent.

Specific examples of the polar solvent include water, alcohol, polyol,glycol ether, 1-methylpyrrolidinone, pyridine, terpineol, butylcarbitol, butyl carbitol acetate, texanol, phenoxypropanol and the like.

Next, the method for producing the fine silver particles of the presentinvention will be described.

A method for producing fine silver particles by present invention isincluding these characteristic steps: a step of making a fluid of rawmaterial and a fluid of reduction; a step of increasing fluidstemperature; a step of reaction, raw material fluid adding into thereduction fluid; a ripeness step of growing the metal particles(especially silver particles) in the fluid; a filtration/washing step ofrepeatedly filtering and washing to remove superfluous organicsubstances; and a drying step of removing moisture from the fluid bydrying.

The filtration step will be described in more detail. In manyconventional fine silver particle reactions, the fine silver particlesfloat into the reaction fluid and cannot be easily separating becausethe fine silver particles completely disperse by primary particles intothe reaction fluid after the reaction.

Such fine silver particles are usually separated by making them sedimentin the fluid by centrifugation.

However, according to the production method of invention, nano-sizedprimary fine particles form a loose agglomeration. So agglomeration sinkin the reaction fluid naturally, the particles can be separated easily.

So the particles can be separated easily with filtration, withoutcentrifugation, using a filter cloth and so on.

This dramatically contributes for increasing of the mass-productivityand in reducing costs.

In the present invention, a step of making a fluid of reduction, a stepof reaction of silver, and the filtration/washing step are carried outas follows.

Specifically, the reduction fluid to be used in a step of making a fluidof reduction includes water, ammonia water, hexanoic acid, and hydrazinehydrate.

In the silver reaction step, an aqueous silver nitrate fluid is added tothis reduction fluid and a reaction is carried out.

In the filtration/washing step, the product obtained in the reactionstep is separated by filtration, and then washed with water.

The ammonia water included in the reduction fluid is added as astabilizing agent to dissolve the acid by the water.

In the silver reaction step, the reaction is carried out by increasingthe temperature to a range preferably between 40° C. and 80° C. in thereaction vessel.

It is more preferred for the reaction, the temperature of the aqueoussilver nitrate fluid, as same temperature as the fluid of reactionvessel, is prepared.

If the temperature in the reaction vessel is not in the above-describedrange, the following problems may occur.

At less than 40° C., the tendency of supersaturation of metal in thereaction fluid increases so nucleation is more promoted, so a proportionof the fine particles are vainly increased.

At more than 80° C., although nucleation is suppressed, particleabnormal growth and particle aggregation tend to be promoted.

Furthermore, at the silver reaction step, the aqueous silver nitratefluid is added preferably at a stretch for the homogeneous reaction inthe fluid.

If the aqueous silver nitrate fluid is not added at a stretch, the fluidbecomes a heterogeneous system, and nucleation and particleagglomeration occur all at once. This may cause to generate non-uniformsilver particles having a large particle distribution.

Therefore, the expression “added at a stretch” is not especiallylimiting when reaction factors, such as the concentration of reductionagent or protection agent, or pH or temperature of reaction fluid, donot essentially change during the period of adding the aqueous silvernitrate fluid.

The hydrazine hydrate can be changing into any reducing agent, when itcan reduce metal.

Hydrazine hydrate can be used with another reducing agent. The reducingagent except of the hydrazine hydrates are illustrated hydrazine, analkali borohydride (NaBH₄), lithium aluminum hydride (LiAlH₄), ascorbicacid, a primary amine, a secondary amine, a tertiary amine and so on.

The specific surface area of fine silver particle measured by the BETmethod of this invention is 5 to 20 m²/g

The surface area of the particles has influence over viscosity of fluiddispersions.

Therefore, the viscosity of fluid dispersions can be controlled byadjusting the surface area of the fine silver particles.

And the surface area can also be controlled by appropriately adjustingthe temperature in reaction fluid, or the amount of the reducing agentor the protection agent.

If BET value is 5 m²/g or less, that means, the proportion of fineparticles may have decreased, or coarse particles may be increasedcaused by aggregation. This means that it is difficult to control theBET value while maintaining the particle size.

If such particles are used, the trouble, such as clogging of the nozzleaperture, and the incidence of unsuitable particles for fineinterconnecting, may be caused according to printing methods.

When the coarse particles are extremely increased; the ratio ofparticles collected by filter-paper during the below-describe fluid flowevaluation may increase.

Furthermore, although the upper limit of BET value is 20 m²/g, this iscurrent limit upper value of the production by this method.

The point of view about this invention is the fine particles can beproduced stably in a large scale or mass production. In the sense ofthat viewpoint, the value of BET is preferred range is 7-20 m²/g, morepreferred range is 10-20 m²/g.

Measurement of the specific surface area by the BET method, afterperforming a pre-treatment in an N₂ atmosphere at 25° C. with theflowing rate of 45 cc/min, was carried out using 4S-U2 manufactured byYuasa Ionics Inc., or equivalent can use for measurement.

The TAP density is preferred range is 0.5 to 5.0 g/cm³.

If the TAP density is lower than 0.5 g/cm³, the compaction of theparticles in the film is insufficient and the film density aftersintering is reduced, so they may cause resistance to deteriorate.

The TAP density is more preferably 1.0 to 4.5 g/cm³, and even morepreferably 1.5 to 4.0 g/cm³.

Measurement of the TAP density of this invention was carried out using ameasurement method described in Japanese Patent Application Laid-OpenNo. 2007-263860.

Next, the silver content will be described.

The composition containing fine silver particles of the presentinvention may also be used in applications such as interconnectionformation in the electronic materials field and the like.

For use of interconnection formation application and the like, thecomposition containing fine silver particles of the present applicationis printed on a substrate by an arbitrary printing method, and thecomposition sinter for changing to the state of silver.

At the time, if the amount of protection agent covering the silver islarge, the amount of protection agent which evaporates in the sinteringstep increases. Consequently, the ratio of contraction volume of thesilver interconnection before and after the sintering is increased whichis not desirable in terms of substrate production.

This is because a defect such as interconnection scaling is occurred.

Thus, the silver content of the composition containing fine silverparticles is as close as possible to 100 mass %, and preferably 95 mass% or more.

If the silver content is less than 95 mass %, the amount of protectionagent covering the silver particles is too large, and that means causingthe volume contraction ratio to increase.

In the present invention, a more preferred range is 96 mass % or more,and more preferred is 96.5 mass % or more.

Measurement of the silver content was carried out by placing acomposition containing dry fine silver particles in an ash tray (squaretype, 50×30×10) to thickness of 1 to 2 mm, sintering silver particle atthe ash tray in muffle furnace (FO310, manufactured by Yamato ScientificCo., Ltd.), and measured the weight after and before sintering, andcalculate the silver content of the particle.

Furthermore, the sintering conditions were that, in the atmosphere, thetemperature was increased from 25° C. to 700° C. at a rate of increaseof 10° C./min, and then the ash tray was allowed to naturally cool toroom temperature.

The crystallite size of the fine silver particles in the presentinvention is in the range of 1 to 30 nm.

If the crystallite size is less than 1 nm, the fine silver particlesessentially may not be present due to progress of sintering. If thecrystallite size is more than 30 nm, ease of sintering may be worse.

So a preferred range of crystalline size is 5 to 20 nm, and a morepreferred range is 9 to 15 nm.

The crystallite size measurement in the present invention was carriedout by a RINT 2000 manufactured by Rigaku Corporation in crystal face of(111). And the condition of measurement were these: a measurement rangeis 40 to 60° at 2., and multiplied times of measurement is six, andX-ray source was Co, and X-ray tube voltage was 40 kV, and X-ray tubecurrent was 30 mA.

The crystallite size was calculated using the Scherrer equationdescribed by the following equation (1). And the “half value width” βwhich is necessary of the calculation of equation, was worked out byabove-mentioned measurement,Dhkl=(K·λ)/(β·cos θ)  (1)

-   -   Here, the respective variables are as follows.

-   D: Crystallite size (nm)

-   λ: Measurement X-ray wavelength (nm)

-   β: Broadening in the diffraction width due to the crystallite

-   θ: Bragg angle of the diffraction angle

-   K: Scherrer constant    -   In the above equation (1), the measurement X-ray wavelength λ is        1.79, and the Scherrer constant K is 0.94.

The characteristic of this invention is having a dispersion step topolar solvent of the fine silver particles composition afterabove-described method which is composed by doing sequentially; a stepof making reaction fluid, a step of reacting of silver, a step offiltration/washing, and a step of drying.

Here, the term “dispersion” refers to a state where the fine particlesare stably present in the polar solvent. Because of leaving at rest,some of the fine particles may be settled, but they are allowable.

By performing such a step, the fine silver particles in agglomeratestate exist stably under aridity. However, this agglomerationdissociates into individual particles, they can exist stably in thepolar solvent including water by an appropriate redispersion treatment.

The term “agglomeration size”, which refers to the particle size on theagglomerated state, will be described.

In the description of the present invention, Coulter average size isused for the agglomeration size.

The composition containing fine silver particles of present inventioncan separate by filtration in virtue of characteristic of agglomeratingduring the reaction.

Therefore, after the filtration/washing step and in the dry conditionafter the drying step, the silver particles are present in anagglomerated state.

In spite of such a dry condition of fine particles, can be redispersedinto the solvent when used for a dispersion or a paste.

To quantify this phenomenon, the dispersed particle size afterredispersion in the solvent was measured by the Coulter method which iswet type laser diffraction.

That is to say, the average particle size measured by the Coulter methodmeans the size of primary particle which had been dried, and isredispersed in the solvent.

The measurement method for average particle size of the Coulter will bedescribed.

The SDBS fluid was prepared by mixing SDBS (Sodium dodecyl benzenesulfonate) with pure water, that mix ratio was 20:100 by mass. And theSDBS was dissolved completely

Then the test sample was prepared by adding the aridity particle intothe SDBS fluid, that mix ratio was 0.9:100 by mass. And the resultantfluid was subjected to an ultrasonic treatment beyond 1 hour.

UT-205S, which manufactured by Sharp Corporation, was used forultrasonic treatment for preparing sample by 100% output power.

LS-230, which manufactured by Beckman Coulter Inc, was used formeasuring of test samples.

An ideal average particle size measured by Coulter method of thisinvention is 5 μm or less. If the average particle size measured byCoulter method is over 5 μm, this means the redispersibility is badwhich is not desirable.

The average particle size measured by Coulter method of this inventionis preferably 3 μm or less, and more preferably 2 μm or less.

Usually, the average particle size measured by Coulter method in thisinvention is about 0.1 to 2 μm. However, this average particle sizedecreases by extending dispersion treatment time.

Therefore, for the nano powder of this invention, the dispersionparticle size can be controlled on the redispersibility treatmentconditions.

When actually forming a paste, the condition of dispersion is strongerthan that of measurement of Coulter method, so an even smaller value canprobably be obtained.

Furthermore, for one of the index for redispersibility measurement ofthe composition of dried fine particles, a method which measures theparticle mass vestige when filtering the dispersion is commonly used.

This is called a “fluid flow test”.

In the present invention, redispersibility was examined of thedispersions, which was prepared for measurement average particle size.The dispersion passes through filter-paper having a retaining particlesize of 1 μm, and the weight of particles on the filter was measured forthe redispersibility evaluation. The redispersibility can be representedas (“The weight of remained particle on the filter paper afterfiltration test”/“The weight of particle into the dispersion beforefiltration test”).

In the present invention, the ratio of survival on the filter paper ispreferably less than 10 mass %, more preferably less than 5 mass %, andstill more preferably less than 1 mass %.

If this value is 10 mass % or more, it means that redispersibility ispoor.

No. 5C Filter Paper (φ 90 mm) which having a retaining particle size of1 μm manufactured by Advantec, was used for the present invention. Thefiltration was carried out by suction filtration using of “Büchnerfunnel”.

If the amount of fluid used in the fluid flow test is too much,collection resulting from clogging of the filter paper is likely tooccur. Therefore, the amount of fluid to be used is preferably 150 cm³or less.

Next, the preservation stability will be described.

The fine silver particles are usually used in a dispersion statedispersed in a solvent.

Thus, in practical view, the pot life in the dispersion state isimportant.

In the present invention, difference of viscosity before and afterpreservation was used for an index of pot life evaluation.

The particle of present invention is agglomerated in dry condition. Thusredispersion of this agglomeration needs to do coping of dispersion.

In this case, the viscosity increases along with separation of theagglomerated particles by the dispersion treatment.

By increase of contact area for disentangling of agglomeration; causedto decrease of free solvent, which caused this phenomenon. But by thevariation with time, the particle was agglomerated again, that caused toincrease of free solvent, so the dispersion viscosity decrease.

Thus, changes of dispersion viscosity can be used as a pot lifeevaluation index.

In the case of no difference of viscosity before and after preservationin this invention, it can interpret that there is no deteriorationdispersibility by preservation. On the other hand, in the case ofconfirming the difference of viscosity before and after preservation, itcan interpret that there is deterioration of dispersibility bypreservation.

In the method for producing silver particle, and producing method forcomposition comprising of such silver particles, it is preferred to usea reaction vessel having a shape and configuration in which uniformstirring can be obtained.

This is because, while the fine silver particles are obtained by areduction reaction, the size of the particles to be obtained is verysmall, which means that localized concentration and pH distribution havea large influence on particle size distribution.

So, using of one concrete example of the producing method of silverparticle of this invention, each of the production steps will bedescribed along the flow of the reaction

<Fluid Preparation Step>

In this step, two kinds of fluid are prepared.

One of the fluids is called “Fluid I” (hereinafter referred to as“reduction fluid”), in which a reducing substance is dissolved. Theother fluid is called “Fluid II” (hereinafter referred to as “rawmaterial fluid”), in which the raw material metal salt (especiallysilver salt) is dissolved.

The reduction fluid was prepared by dissolving following materials intopure-water uniformly; the dissolving material was above-describedreducing agent, protection agent, ammonia water acting as a stabilizingagent

The raw material fluid is obtained by dissolving crystals of the metalsalt in pure water.

<Temperature Increasing Step>

After both the fluids have been prepared, the temperature of the fluidswas boosted to the reaction temperature by using a water bath or aheater.

It is preferred to heat up in a similar manner both the reduction fluidand the raw material fluid; this has the advantage of preventing thereaction from being non-uniform during the reaction. In addition, thiscan be achieving the uniformity of the particles.

At this point, the target temperature (hereinafter referred to as“reaction temperature”) of heating is in the range of 40 to 80° C.

<Silver Reaction Step>

At the time the temperature of both fluids has been attained to thetarget temperature, the raw material fluid is added to the reductionfluid.

From the view of reaction uniformity, it is preferred to add at a burst,while taking account to the no-bumping.

<Aging Step>

The stirring, after the mixing of the reaction fluids, had beencontinued for about 10 to 30 minutes until completing the growth of theparticles.

The confirmation of end-point of reaction practiced as follows; samplingliquid from the reaction fluid, and dropping hydrazine into the samplingfluid, and confirming whether a reaction of unreduced silver occurs.

<Filtration/Washing Step>

The obtained slurry can separate to solid and fluid by a filtrationmethod.

As the filtration apparatus, a conventional apparatus can beappropriately employed.

“Büchner funnel” spread with filter paper can be used for a small-scalereaction of a few liters.

The kind of filter paper that can be used at this point is notespecially limited. Even filter paper having a retaining particle sizeof several microns can be used.

For a large-scale reaction of tens of liters or more, a filter press canbe used.

The washing step is carried out by adding pure water to a cake obtainedfrom the filtration step, and then again filtering that pure water (thestate of slurry).

Furthermore, the obtained slurry can be separated to solid and fluid onforcibly sinking particles by using a centrifugal separator.

In this case, the centrifugal separation is carried out by operating at3,000 rpm for 30 minutes.

After the solid/fluid separation, do away with the supernatant liquid,and pure water is added, and the fluid is dispersed for 10 minutes by anultrasonic disperser.

The washing step as has been above-mentioned for removing superfluousorganic substances adhering to the particles is carried out three times;the step of centrifugal separation, the step of doing away with thesupernatant liquid, the step of adding of pure water, and the step ofdispersion by ultrasonic.

<Drying Step>

A dried metallic lump was obtained, by drying-step of 12 hours at 60°C., from metal lump (silver lump) made by former steps,

EXAMPLES

The examples will be explained in detail as follows.

Working Example 1

A one liter beaker was used for a reaction vessel.

The stir stick that equipped the stir slats was placed on the center ofthe reaction vessel.

A thermometer for monitoring the temperature was placed in the reactionvessel.

In addition, a nozzle was constituted so as to supply nitrogen into thefluid from the underneath.

First, 273 g of water was charged into the reaction vessel, and nitrogenwas supplied from the underneath of the reaction vessel between 600seconds 500 mL/min (flow rate) for dissolved oxygen removing.

Nitrogen was then supplied at a flow rate of 500 mL/min from an upperportion of the reaction vessel to form a nitrogen atmosphere in thereaction vessel.

The rotation speed of the stir stick was adjusted from 280 to 320 rpm.

Then, the fluid temperature of the reaction vessel was controlled for60° C.

7.5 g of ammonia water (the containing of ammonia is 30 mass %) wasadded into the reaction vessel, and the fluid was stirred for 1 minuteto form the homogeneous system.

7.5 g of hexanoic acid as a protection agent (guaranteed reagent,manufactured by Wako Pure Chemical Industries Ltd.) (The weight iscorresponding to 2.01 equivalents of the silver) was added into thereaction vessel. And the fluid was stirred 10 minutes for dissolving theprotection agent.

A different reagent each for working examples and comparative exampleswas used as the protection agent.

Then, 20.9 g of hydrazine hydrate (the containing of hydrazine is 50mass %, manufactured by Otsuka Chemical Co., Ltd) was added into thereaction vessel as reduction fluid.

Here, “equivalents of the silver” in this description mean equal to themolar ratio of silver versus the hexanoic acid. So the “2.01equivalents” indicate that the hexanoic acid/silver molar ratio is 2.01.

In another vessel, 36 g of silver nitrate crystal was dissolved in 175 gof water (guaranteed reagent, manufactured by Wako Pure ChemicalIndustries Ltd.), liquid called as “raw material fluid”. And this liquidtemperature was controlled for 60° C.

Then, the reduction reaction was caused from the act that “raw materialfluid” added into “reduction fluid” at a burst.

The stirring was continued while maturing for 10 minutes.

Then, the stirring was stopped, and the fluid underwent afiltration/washing step and a drying step for obtaining a lump of finesilver particles.

When particles obtained at the stage where the washing step was finishedwere observed by using a TEM, as illustrated in FIG. 4, it was foundthat particles was attached with the hexanoic acid were obtained. Theseparticles were fine and comparatively uniform size, and an averageparticle size was 14 nm.

The primary particle average size according to the TEM was determined bymeasuring the circle equivalent diameter of 300 or more particlespresent in the field of view at 174,000 times magnification, and thenhad calculated of the number-average diameter of those particles.

As understood from the SEM image shown in FIG. 1, dried particles wereobtained in form of agglomerate. However, it observed that theagglomerate has many very fine particles when studied carefully underhigh magnification,

In addition, “fluid flow test” and “particle size measurement by Coultermethod” were executed. In this case, the measurement sample was preparedas follows; 20 g of SDBS and 0.9 g of dry fine silver particles wereadded into 100 g of pure water, and the mixture was treated byultrasonic dispersion for 1 hour.

Working Example 2

The reaction of working example 1 was carried out under the sameconditions except that the reaction temperature was changed to 50° C.

The BET value in this case was 7.1 m²/g.

Comparative Example 1

Silver particles were synthesized in the same manner as in workingexample 1, except that the hexanoic-acid was changed to heptanoic-acidhaving seven carbon atoms.

Furthermore, a dispersion was prepared under the same conditions inworking example 1. And “fluid flow test” and “particle size measurementby Coulter method” were executed.

Comparative Example 2

7.5 g of ammonia water (the containing of ammonia is 30 mass %) wasadded into the reaction vessel, and the fluid was stirred for 1 minuteto form the homogeneous system.

7.5 g of hexanoic acid as a protection agent (guaranteed reagent,manufactured by Wako Pure Chemical Industries Ltd.) (The weight iscorresponding to 2.01 equivalents of the silver) was added into thereaction vessel. And the fluid was stirred 10 minutes for dissolving theprotection agent.

Then, 20.9 g of hydrazine hydrate (the containing of hydrazine is 50mass %, manufactured by Otsuka Chemical Co., Ltd) was added into thereaction vessel as reduction fluid.

In another vessel, 36 g of silver nitrate crystal was dissolved in 175 gof water (guaranteed reagent, manufactured by Wako Pure ChemicalIndustries Ltd.), liquid called as “raw material fluid”. And this liquidtemperature was controlled for 60° C.

Then, a reduction reaction was carried out by gradually adding the rawmaterial fluid to the reduction fluid at an addition rate of 5 mL/min(requiring 35 minutes until the addition was finished) using a tubepump.

The stirring was continued while maturing for 10 minutes.

Then, the stirring was stopped, and the fluid underwent afiltration/washing step and a drying step for obtaining a lump of finesilver particles.

Comparative Example 3

Silver particles were produced in the same manner as working example 1,except that heptanoic acid was not added.

Next, to confirm the pot life of the composition containing fine silverparticles of this invention, working examples 20 to 23 were carried outas follows.

Working Example 20

Terpineol was added to the fine silver particles of this invention untilthe concentration of silver was 60 mass %, and dispersed to obtain thepaste.

The dispersion was carried out by manually stirring, and then mechanicaldispersion treatment was executed by “Awatori-Rentaro AR-250”manufactured by Thinky Corporation by a mixing mode for 1 minute, andmore dispersion treatment was executed by a triple roll mill.

This paste was placed in an incubator at 25° C. Redispersion wasexecuted, in that day and after keeping in incubator on the 7 days and13 days. And the viscosity of these paste were measured by rheometer.The dispersion treatment was executed by “Awatori-Rentaro AR-250” by amixing mode for 1 minute, and more dispersion treatment was carried outby manually stirring, and more mechanical dispersion treatment wasexecuted by “Awatori-Rentaro AR-250” by a mixing mode for 1 minute.

Working Example 21

The process of working example 20 was carried out under the sameconditions except that the solvent was changed to 2-phenoxy-1-propanol(hereinafter referred to as “Downol”).

Working Example 22

The process of working example 20 was carried out under the sameconditions except that the solvent was changed to Texanol.

Working Example 23

The process of working example 20 was carried out under the sameconditions except that the solvent was changed to butyl carbitol acetate(hereinafter referred to as “BCA”).

In Working Examples 30 to 34 inquired into the influence for pot lifefrom the dispersion agent.

Working Example 30

The process of working example 20 was carried out under the sameconditions except that 5 mass % of Disper BYK 2001 (manufactured by BYKJapan KK) with respect to silver was used as the dispersion agent.

Working Example 31

The process of working example 30 was carried out under the sameconditions except that the solvent was changed to Downol.

Working Example 32

The process of working example 30 was carried out under the sameconditions except that the solvent was changed to Texanol.

Working Example 33

The process of working example 30 was carried out under the sameconditions except that the solvent was changed to BCA.

Working Example 34

The process of working example 30 was carried out under the sameconditions except that the solvent was changed to γ-butyrolactone.

The experiment conditions are shown in Table 1.

The results of the Coulter size measurement and the fluid flow testcarried out in Working Example 1 and Comparative Example 1 are shown inTable 2.

Furthermore, the results on Working Examples 20 to 34 are shown in Table3 and FIGS. 8 and 9.

TABLE 1 Raw Material Organic Protection Agent Addition Method ProtectionAg Added Equivalents with Into Reduction Reaction Agent Amount respectto Ag Solution Temperature Example 1 Hexanoic 0.05 mol/L 2.01eq All atonce 60° C. acid Example 2 Hexanoic 0.05 mol/L 2.01eq All at once 50° C.acid Comparative Heptanoic acid 0.05 mol/L 2.01eq All at once 60° C.Example 1 Comparative Heptanoic acid 0.05 mol/L 2.01eq Continuous 60° C.Example 2 addition Comparative None 0.05 mol/L — All at once 60° C.Example 3

FIGS. 1 to 3 show photographs of each sample taken by a scanningelectron microscope (FE-SEM). FIGS. 4 to 7 show photographs of eachsample taken by a transmission electron microscope (TEM).

FIG. 1( a) is at a magnification of 10,000 times, and FIG. 1( b) is at amagnification of 100,000 times.

The double-headed arrows in the photographs represent 3.0 μm for FIG. 1(a) and 300 nm for FIG. 1( b).

This is the same for all of FIGS. 1 to 3.

In Example 1 (FIG. 1), it can be observed under SEM observation at10,000 times magnification that fine particles with a comparativelyuniform size were gathered together.

Even under observation at 100,000 times magnification, the particle sizeseemed very uniform, and the particles are each produced in an isolatedshape. In contrast, it can be seen that for the particles of ComparativeExample 2 (FIG. 2), particles agglomeration or particle sintering hasfrequently occurred, and that the uniformity of the particle size is notobtained.

At the particles of Comparative Example 3 (FIG. 3), which were formedwithout adding the protection agent, the particles themselves looked asthough they have been sintered. Furthermore, it can be seen that theconfirmed primary particles are both large and irregular.

FIG. 4 is TEM photograph of particle state obtained from Working Example1.

FIG. 4( a) is at 30,000 times magnification, and FIG. 4( b) is at174,000 times magnification. A 500 nm reference line was put in thephotograph of FIG. 4( a), and a 100 nm reference line in FIG. 4( b).

It can be perceived that at 30,000 times magnification photograph, roughparticles formed by sintering are not present.

On the other hand, particles formed agglomeration which seems lumps whenobserved at 30,000 times magnification. And in the observation at174,000 times magnification, these particles which formed agglomerationwere separate and independent.

Furthermore, it can be perceived that spherical particles having acomparatively uniform particle size were obtained.

On the other hand, in Comparative Example 1 (FIG. 5), similar to theworking examples, it was confirmed that the particles were agglomeratedto each other, and that the individual particles were independent.

However, when each particle was confirmed, it can be perceived that theparticle size was not uniform, and the particle shape was alsonon-uniform.

It can be also said in Comparative Example 2 (FIG. 6), where it can beperceived that the particle size dispersion was more conspicuous. In theExample, particle had a wide size region of from several nanometers totens of nanometers.

In Comparative Example 3 (FIG. 7), it is perceived to generate theparticle that forms the infinite form and enormous size by sintering.Consequently, it can be understood that the presence of the protectionagent is necessary for securing the independence of the particles.

The above results suggest that fine particles of 100 nm or less cannotbe produced unless a protection agent is in existence regardless ofheptanoic-acid or hexanoic-acid.

That is to say, the above-results suggest that without the existence ofa protection agent, the growth of silver and the sintering of eachparticle due to additional reduction reactions after the particles whichwill serve as the core are formed in the reaction vessel cannot bestopped. And the results suggest that the lack of the protection agentcaused disorderly growth and developing.

In other words, if the reduction reaction of silver is executed underthe existence of the protection agent, it can be said that theprotection agent is adhere or bonding with surface of the reactionproduct particle.

Next of view, by comparing of working example 1 and Comparative Example2, it can be perceived that Comparative Example 2 has more amounts ofsmall size particles.

It can guess to be a lot of small particles at high probability, notonly from the TEM photograph at 174,000 times magnification, but alsofrom the SEM photograph at 10,000 times magnification which seemed to bean indeterminate shape.

Namely, in the SEM photograph 10,000 times magnification, silverparticles having a few nm diameters can not observe by each independentparticle but as continuous matter.

It is thought that the protection agent have an effect to inhibit aboutsilver growth at the surface of the silver particle when the reductionreaction has occurred by injecting of the “raw material fluid” into thereaction vessel. Thus, the difference between hexanoic acid andheptanoic acid can be considered to be an important factor indetermining whether particles having a uniform particle shape andparticle size distribution can be produced.

More specifically, the results show that heptanoic acid was insufficientto form particles having a uniform particle shape. Furthermore, whylarge particles were also formed when heptanoic acid was continuouslyadded can be thought to be as follows. Namely, because the heptanoicacid acting as the protection agent was consumed in the initial reactionstage, there was not enough heptanoic acid to protect against reactionstowards the end of the addition of the aqueous silver nitrate fluid.

Therefore, it was learned that to obtain a comparatively uniformparticle size, it is preferred to add the fluid at a burst.

Thus, in addition to using hexanoic acid as the protection agent, it isthought that the addition at a dash of the silver nitrate aqueous fluidcontribute in part for obtaining uniformly fine silver particle at levelof several tens of nm.

Next, at referring of Table 2, from the result of working example 1 thefine silver particle and composition was obtained by using hexanoicacid, has small Coulter average size (0.201 μm), and has smallcollection efficiency of the particle on the filter paper. It means thatthe particle was the agglomeration on the state of drying. But, once theparticles are added into the solvent, can redisperse.

TABLE 2 Organic Coulter Size Liquid Protection (Average) Passing Agent(μm) Test (%) Example 1 Hexanoic acid 0.201 0.23 Comparative Heptanoicacid 9.34 66.6 Example 1

In contrast, the fine silver particle in comparative Example 1 the finesilver particle and composition was obtained by using heptanoic acid,has a very large Coulter average size, and has large collectionefficiency (66.6 mass %) of the particle on the filter paper.

Consequently, it is suggested that when hexanoic acid is used for theprotection agent, the redispersibility of particles of drying state whenadding into the solvent is markedly improved.

Next, at referring of Table 3 and FIG. 8.

The viscosity described in Table 3 is a value measured using theRheoStress RS600 (Haake). The condition at the measurement is the shearrate dγ/dt (1/s) was 3.038 at 25° C.

If the fine silver particle is dispersed, the increase of viscosityoccurred by the degree of the dispersion, by lacking of the free solventcaused by increasing the contact area with the solvent by the largeamount of surface area.

Namely, the degree of viscosity is able to use for index ofredispersibility.

TABLE 3 Dispersion Viscosity Solvent Agent Same Day 7th Day 13th Day14th Day 21st day Example 20 Terpineol No 12800 11400 9660 — — Example21 Downol No 9000 7220 7170 — — Example 22 Texanol No 4960 3950 3240 — —Example 23 BCA No 1460 577 1320 — — Example 30 Terpineol Yes 36900 21900— 25900 24200 Example 31 Downol Yes 64300 67400 — 80000 79900 Example 32Texanol Yes 20300 14200 — 13700 14400 Example 33 BCA Yes 12200 9340 —12300 10600 Example 34 γ-Butyrolactone Yes 12000 1890 —  3880 —

FIG. 8 is plotted the value of viscosity by Working Examples 20 to 23extracted from Table 3.

The horizontal axis represents the number of preservation days, and thevertical axis represents the viscosity.

As described above, a high viscosity means that dispersibility is high.

In all of the examples, viscosity tended to decrease over time, anddispersibility tended to deteriorate.

However, Downol and terpineol have a high viscosity originally, so thatdispersibility can be considered to be high.

Next, the effect of the dispersion agent in the paste will be described.

In practically, the pot life of the paste is an important feature.

Therefore, Examples 30 to 34 were produced by adding a solvent and adispersion agent to the composition containing fine silver particles ofthe present invention.

FIG. 9 shows the value of viscosity by Working Examples 30 to 34extracted from Table 3.

Due to the effect of the dispersion agent, all of the examples showed ahigher viscosity from the initial viscosity than for when a dispersionagent was not used.

Consequently, the effect of adding a dispersion agent into the paste wasrecognizable.

Furthermore, for all of Examples 30 to 34, the value of viscosity after7 days was a constant.

Thus, it was appreciated that including a dispersion agent in the pasteimproves the pot life.

INDUSTRIAL APPLICABILITY

According to the present invention, a composition containing fine silverparticles having excellent redispersibility for a solvent, and adispersion using that composition containing fine silver particles, canbe obtained. Consequently, the present invention can be preferablyutilized in various applications using such composition or dispersioncontaining fine silver particles.

For example, the present invention can be utilized in variousapplications, such as electrode formation for “FPD/solar cells/organicEL”, “RFID wire formation”, “a fine trench”, “wires which are embeddedin via hole contact hole or the like”, “a colorant for automobile orship coatings”, “a carrier on which is adsorbed a biochemical substancefor the medical, diagnostic, or biotechnology fields”, “an antimicrobialcoating utilizing an antimicrobial effect”, “a catalyst”, “a conductiveadhesive”, “a conductive paste formed by mixing with a resin or aflexible printed circuit board using such a conductive paste”, “ahigh-flexibility shield”, “a capacitor” and the like.

According to the present invention, a composition in which fine silverparticles are uniformly monodispersed in a polar solvent can beobtained. Therefore, the present invention can be utilized inapplications (for example, a reflective film) in which it is thoughtthat effects can be obtained due to a uniform particle size.

1. A composition containing fine silver particles, comprising silverparticles which are bonded to a linear fatty acid having six or lesscarbon atoms, wherein a primary particle average size measured from TEMobservation is in a range of 1 to 100 nm, and a specific surface areameasured by the BET method of the silver particles is 7 to 20 m²/g. 2.The composition containing fine silver particles according to claim 1,wherein an average particle size measured by Coulter method when adispersion formed from 20 parts by mass of sodium dodecyl benzenesulfonate and 0.9 parts by mass of the fine silver particles in a drystate with respect to 100 parts by mass of pure water is subjected to a1 hour ultrasonic treatment is less than 5 μm.
 3. The compositioncontaining fine silver particles according to claim 1, wherein, whenpassed through filter-paper having a retaining particle size of 1 μm, apercentage ratio of particle mass collected on the filter paper toparticle mass included in dispersion is less than 10 mass %.
 4. Thecomposition containing fine silver particles according to claim 2,wherein, when passed through filter-paper having a retaining particlesize of 1 μm, a percentage ratio of particle mass collected on thefilter paper to particle mass included in dispersion is less than 10mass %.
 5. A composition containing fine silver particles, comprisingsilver particles which are bonded to a linear fatty acid having six orless carbon atoms, wherein the linear fatty acid is hexanoic acid, aprimary particle average size measured from TEM observation is in arange of 1 to 100 nm, and a specific surface area measured by the BETmethod of the silver particles is 7 to 20 m²/g.
 6. The compositioncontaining fine silver particles according to claim 5, wherein anaverage particle size measured by Coulter method when a dispersionformed from 20 parts by mass of sodium dodecyl benzene sulfonate and 0.9parts by mass of the fine silver particles in a dry state with respectto 100 parts by mass of pure water is subjected to a 1 hour ultrasonictreatment is less than 5 μm.
 7. The composition containing fine silverparticles according to claim 5, wherein, when passed throughfilter-paper having a retaining particle size of 1 μm, a percentageratio of particle mass collected on the filter paper to particle massincluded in dispersion is less than 10 mass %.
 8. The compositioncontaining fine silver particles according to claim 6, wherein, whenpassed through filter-paper having a retaining particle size of 1 μm, apercentage ratio of particle mass collected on the filter paper toparticle mass included in dispersion is less than 10 mass %.
 9. Acomposition containing fine silver particles, comprising silverparticles which are bonded to a linear fatty acid having six or lesscarbon atoms, wherein the composition is dispersed in a polar solvent, aprimary particle average size measured from TEM observation is in arange of 1 to 100 nm, and a specific surface area measured by the BETmethod of the silver particles is 7 to 20 m²/g.
 10. The compositioncontaining fine silver particles according to claim 9, wherein anaverage particle size measured by Coulter method when a dispersionformed from 20 parts by mass of sodium dodecyl benzene sulfonate and 0.9parts by mass of the fine silver particles in a dry state with respectto 100 parts by mass of pure water is subjected to a 1 hour ultrasonictreatment is less than 5 μm.
 11. The composition containing fine silverparticles according to claim 9, wherein, when passed throughfilter-paper having a retaining particle size of 1 μm, a percentageratio of particle mass collected on the filter paper to particle massincluded in dispersion is less than 10 mass %.
 12. The compositioncontaining fine silver particles according to claim 10, wherein, whenpassed through filter-paper having a retaining particle size of 1 μm, apercentage ratio of particle mass collected on the filter paper toparticle mass included in dispersion is less than 10 mass %.
 13. Acomposition containing fine silver particles, comprising silverparticles which are bonded to a linear fatty acid having six or lesscarbon atoms, wherein the linear fatty acid is hexanoic acid, thecomposition is dispersed in a polar solvent, a primary particle averagesize measured from TEM observation is in a range of 1 to 100 nm, and aspecific surface area measured by the BET method of the silver particlesis 7 to 20 m²/g.
 14. The composition containing fine silver particlesaccording to claim 13, wherein an average particle size measured byCoulter method when a dispersion formed from 20 parts by mass of sodiumdodecyl benzene sulfonate and 0.9 parts by mass of the fine silverparticles in a dry state with respect to 100 parts by mass of pure wateris subjected to a 1 hour ultrasonic treatment is less than 5 μm.
 15. Thecomposition containing fine silver particles according to claim 13,wherein, when passed through filter-paper having a retaining particlesize of 1 μm, a percentage ratio of particle mass collected on thefilter paper to particle mass included in dispersion is less than 10mass %.
 16. The composition containing fine silver particles accordingto claim 14, wherein, when passed through filter-paper having aretaining particle size of 1 μm, a percentage ratio of particle masscollected on the filter paper to particle mass included in dispersion isless than 10 mass %.
 17. A composition containing fine silver particles,comprising silver particles which are bonded to a linear fatty acidhaving six or less carbon atoms, wherein the composition is dispersed inany one of or both of water and terpineol, a primary particle averagesize measured from TEM observation is in a range of 1 to 100 nm, and aspecific surface area measured by the BET method of the silver particlesis 7 to 20 m²/g.
 18. The composition containing fine silver particlesaccording to claim 17, wherein an average particle size measured byCoulter method when a dispersion formed from 20 parts by mass of sodiumdodecyl benzene sulfonate and 0.9 parts by mass of the fine silverparticles in a dry state with respect to 100 parts by mass of pure wateris subjected to a 1 hour ultrasonic treatment is less than 5 μm.
 19. Thecomposition containing fine silver particles according to claim 17,wherein, when passed through filter-paper having a retaining particlesize of 1 μm, a percentage ratio of particle mass collected on thefilter paper to particle mass included in dispersion is less than 10mass %.
 20. The composition containing fine silver particles accordingto claim 18, wherein, when passed through filter-paper having aretaining particle size of 1 μm, a percentage ratio of particle masscollected on the filter paper to particle mass included in dispersion isless than 10 mass %.
 21. A composition containing fine silver particles,comprising silver particles which are bonded to a linear fatty acidhaving six or less carbon atoms, wherein the linear fatty acid ishexanoic acid, wherein the composition is dispersed in any one of orboth of water and terpineol, a primary particle average size measuredfrom TEM observation is in a range of 1 to 100 nm, and a specificsurface area measured by the BET method of the silver particles is 7 to20 m²/g.
 22. The composition containing fine silver particles accordingto claim 21, wherein an average particle size measured by Coulter methodwhen a dispersion formed from 20 parts by mass of sodium dodecyl benzenesulfonate and 0.9 parts by mass of the fine silver particles in a drystate with respect to 100 parts by mass of pure water is subjected to a1 hour ultrasonic treatment is less than 5 μm.
 23. The compositioncontaining fine silver particles according to claim 21, wherein, whenpassed through filter-paper having a retaining particle size of 1 μm, apercentage ratio of particle mass collected on the filter paper toparticle mass included in dispersion is less than 10 mass %.
 24. Thecomposition containing fine silver particles according to claim 22,wherein, when passed through filter-paper having a retaining particlesize of 1 μm, a percentage ratio of particle mass collected on thefilter paper to particle mass included in dispersion is less than 10mass %.